How Severe Weather Indices Predict Storm Intensity

Top 10 Severe Weather Indices Meteorologists Rely On

Below are ten widely used indices with what each measures and typical thresholds or interpretation.

1. CAPE (Convective Available Potential Energy)

   • What it measures: Amount of buoyant energy available to an air parcel (J/kg).
   • Interpretation: 0–1000 marginal, 1000–2500 moderate, 2500–3500 very unstable, >3500 extreme.

2. Shear (0–6 km bulk shear / effective bulk shear)

   • What it measures: Vertical wind change (speed/direction) through the lowest kilometers (kt).
   • Interpretation: 25–40 kt supportive of organized storms; ≥40 kt strongly supportive of supercells.

3. SRH (Storm-Relative Helicity, 0–1 km and 0–3 km)

   • What it measures: Streamwise vorticity available to updrafts (m2/s2).
   • Interpretation: 0–100 low, 100–300 favorable for supercells, >300 very favorable for rotating storms/tornadoes.

4. LI (Lifted Index)

   • What it measures: Parcel temperature difference when lifted to 500 mb (°C).
   • Interpretation: 0 to −2 marginal, −3 to −5 moderate, ≤−6 very unstable (more negative = greater instability).

5. CIN (Convective Inhibition)

   • What it measures: Energy barrier preventing parcel ascent (J/kg).
   • Interpretation: Low CIN (<15–50 J/kg) allows storms; strong CIN suppresses convection until broken by forcing.

6. SWEAT (Severe Weather Threat Index)

   • What it measures: Composite of low‑level moisture, instability, winds and directional shear.
   • Interpretation: ~150–300 slight chance, >300 severe possible, >400 tornado potential (guidance values).

7. EHI (Energy‑Helicity Index)

   • What it measures: Combines CAPE and SRH to assess rotating updraft/tornado potential.
   • Interpretation: EHI >1 indicates supercell potential; larger values increase tornado/higher intensity risk.

8. BRN (Bulk Richardson Number)

   • What it measures: Ratio of instability (CAPE) to shear (0–6 km shear).
   • Interpretation: ~10–50 favorable for supercells; very low (<10) or high (>50) favor other storm modes.

9. TOTL/Total Totals / K‑Index

   • What they measure: Simple thermodynamic indices for thunderstorm/heavy rain potential (temperature and dewpoint differences).
   • Interpretation: Total Totals >50 suggests significant thunderstorm potential; K‑Index: 15–25 small, 26–39 moderate, ≥40 high convective potential.

10. LCL / LFC heights (Lifted Condensation Level / Level of Free Convection)

    • What they measure: LCL — approximate cloud base height; LFC — level where parcel becomes buoyant (m).
    • Interpretation: Lower LCL (<1000 m) favors tornado genesis; lower LFC (<1500–2000 m) favors easier convective initiation.

Notes — practical guidance (concise):

• No single index guarantees severe weather; forecasters consider multiple indices plus forcing, moisture, timing, and mesoscale boundaries.
• Thresholds vary by region/season; use values as guidance, not absolute rules.

If you want, I can expand any index with the formula, calculation example, and typical operational thresholds for the continental U.S. (choose one or I’ll pick CAPE).